In the sorting of lumber according to its bending stiffness, the process most commonly used in high speed production facilities in North America bends the lumber in a machine with a series of rollers as the lumber passes longitudinally through the machine. Background information on such testing processes and equipment can be found in U.S. Pat. Nos. 3,194,063 (McKean) and 3,196,672 (Keller), which are hereby incorporated into this disclosure by reference. A commercial implementation of Keller's patent is entitled "CLT--CONTINUOUS LUMBER TESTER", produced and sold by Metriguard, Inc., Pullman, Wa. For brevity, this described machine will be identified as the "CLT".
To achieve bending stiffness measurements, the CLT utilizes two bending sections; in the first, the lumber is bent downward by a fixed amount, and in the second, the lumber is bent upward by a fixed amount. The force measurements from the two bending sections are averaged to give a result that is independent of deviations from straightness in the lumber. In each bending section, the lumber bending deflection is maintained substantially constant. Lumber sorted according to its bending stiffness together with some visual restrictions and offline quality control procedures can be marketed as Machine Stress Rated (MSR) lumber.
In the CLT, bending deflection of the lumber is caused by a load beam assembly with a pivot on one end and a force measuring transducer on the other. At a point along the load beam assembly, a load point applies the force required to bend the lumber by a prescribed amount. Or, one can say that the lumber applies a force to the load beam assembly at the load point. In the CLT example, the load point consists of a line contact that a load roller makes with the lumber, where the load roller is mounted by bearings to the load beam assembly. At high speeds, constant lumber deflection and fixed position of the load beam assembly are important because that substantially reduces errors at the force measuring transducer that are caused by accelerations of the load beam assembly.
In practice, even though care is taken to reduce the movements, and hence accelerations of the load beam assembly, some extraneous accelerations remain due to various forces that result from the lumber moving through the machine and from external sources, such as vibrating machinery in the area. These accelerations are most pronounced in the second of the CLT's two bending sections. In the second bending section, the load beam assembly is mounted to a bridge frame that is itself suspended from lower clamp roller bearing points that move up and down slightly as lumber enters and exits the bending section. The movement is required to allow the lower clamp rollers to properly clamp and reference the lumber upwardly against the upper clamp rollers at the ends of the bending span. Proper setup and careful control of lumber thickness minimize the motion required for clamping, but even after all care is exerted, some accelerations remain. Because of the inertia of the load beam assembly, these accelerations show up in the measurement as noise superimposed on the desired output signal and hence they reduce the accuracy and resolution with which bending stiffness measurements can be made.
In 1978, Metriguard Inc. introduced an "inertial compensation" system which uses an accelerometer mounted at the base of the force measuring transducer in the second bending section to measure accelerations at the base of the force measuring transducer in directions along its axis and electronically cancel the acceleration-induced noise. This system improves the performance of the force measurement system but does not satisfactorily remove the noise over the conditions of vibration and shock inputs encountered in high speed lumber production facilities. The reason is that the load beam assembly is mounted and referenced to the CLT machine frame at two points and hence can have two independent components of mechanical excitation which are not completely resolved and corrected by measurements from the one accelerometer as presently configured.
It is the objective of this disclosure to describe a linear model of the load beam assembly and to define a selected location for a single compensating accelerometer whose signal can be used to satisfactorily compensate for the effects of acceleration inputs at both reference points where the load beam assembly is mounted. It will be seen that this compensation performs at all frequencies for which the model is valid.
The model consists of a rigid beam having a load point. The beam is supported by two compliant supports at two reference points along it. Mechanical inputs to the rigid beam are through the load point and through the compliant supports. Using superposition principles allows removing the input at the load point to analyze the effects of acceleration inputs at the compliant supports. It happens that a selected location on the beam can be determined such that a signal proportional to the acceleration at just this one point can be used to compensate for acceleration inputs at both compliant supports.
The rigid beam and the two compliant supports form a mechanical system that is excited by acceleration inputs at the compliant supports. For the analysis and computations, a reference direction is defined and only those components of force applied at the load point, force measured at the force transducer, and acceleration inputs that are aligned with the reference direction are considered. Mechanical linkages and/or gearing could be used to avoid this alignment, but here the idea is presented in simplest terms, and the preferred embodiment is described in these terms. In the CLT which provides the framework for the preferred embodiment, the reference direction is vertical.
Further relating the model to the CLT, the two compliant supports for referencing the load beam assembly to the CLT frame are comprised of a series combination of a force transducer and some compliance at one end of the load beam assembly, and a parallel combined effect of two coaxial stub shafts that form a pivot support at the other end of the load beam assembly. The stub shafts and their mounting to the CLT frame are not perfectly rigid, and thus they are best modeled as a compliant support. It will be clear that the first compliant support, being the series combination of the force transducer and load beam end, has force proportional to the compression of the first compliant support and that this force is also proportional to the component of compression for the force transducer alone.
The four state variables used in the analysis of the model are translational velocity of the rigid beam in the reference direction, rotational velocity of the beam about an axis perpendicular to a plane containing both the longitudinal axis of the beam and the reference direction, and compressions in the reference direction of both compliant supports, treating them as springs. Accelerations in the reference direction at the two compliant supports are treated as two input variables. One output variable is taken as the compression at the first compliant support, and a second output variable is taken as the acceleration in the reference direction of a selected location on the rigid beam. Using this model, it can be shown that these two output variables are identical, except for a known constant, over all combinations of the acceleration inputs, provided that the selected location of the output acceleration is chosen according to the present disclosure.
An electrical signal derived in the existing CLT design is proportional to the compression of the force transducer and is thus proportional to the force at the first compliant support. A desired component of this signal, because of the geometry of the load beam assembly, is proportional to the force in the reference direction applied by the lumber to the load roller; but a noise component caused by extraneous acceleration inputs at the compliant supports is also evident. It will be clear from this disclosure that a measure of acceleration in the reference direction at the selected location on the load beam assembly can be used to derive an electrical signal which, when combined with the electrical signal derived from the force transducer, will cause the acceleration-induced noise to be cancelled.
In the CLT at present, the acceleration induced noise is reduced partially by the inertial compensation technique introduced by Metriguard, Inc. in 1978 and further by electronic low-pass filtering of the signal. The filtering reduces the signal bandwidth and acts to attenuate frequency components of the signal including the noise above a cutoff frequency. If the acceleration induced noise component of the force transducer signal consists primarily of frequencies above the range of load point force signal frequencies of interest, then this is a satisfactory approach. In fact, until 1978, no compensation for the effects of these acceleration inputs was used. But, as accuracy and speed requirements have increased, it has become necessary to improve the measurement.
The inertial compensation introduced in 1978 caused a significant improvement, and as a result the system was retrofitted to most of the existing CLT machines and has become standard on all new ones. Since 1978, even higher machine speeds have become common, and the accuracy requirements have become more stringent. The high speeds require the ruggedness and hence massiveness of the present load beam assembly in order to survive; although it is recognized that reductions in mass would increase the frequencies of the acceleration induced noise component of the force transducer signal and hence make the noise more easily removable by low-pass filtering methods. Reducing the signal bandwidth by reducing the cutoff frequency of the low-pass filter would reduce the noise, but this would also reduce the spatial resolution of the force measurement along the lumber. From the correspondence between time and distance along the lumber as the lumber moves through the machine, it is clear that spatial resolution of the force measurement decreases as the machine speed increases, and to gain back this spatial resolution, it is necessary to increase the signal bandwidth. A summary of conflicting requirements is:
Reduce mass of load beam assembly to increase frequency content of acceleration induced noise.
Increase mass of load beam assembly to improve the ruggedness and durability of the machine at high production speeds.
Reduce cutoff frequency of electronic low-pass filter to remove more of the acceleration induced noise.
Increase cutoff frequency of electronic low-pass filter to improve the spatial resolution of the force measurement along the length of the lumber.
Decrease machine speed to improve the spatial resolution of the force measurement along the length of the lumber and reduce the magnitude of the acceleration excitation inputs.
Increase machine speed to reduce production costs and improve profitability.
The above requirements have led to engineering tradeoffs that are probably close to being optimum for the present system. But one fact is inescapable; speeds are increasing, and further improvements are necessary. The inertial compensation system of 1978 helped the situation by removing some of the acceleration induced noise; however, that system measures the acceleration at one point on the CLT bridge frame, and that does not compensate for enough of the induced noise.
The present disclosure shows that an optimum location exists at a selected location on the load beam assembly, such that measuring acceleration at the selected location makes possible significant further improvement in the reduction of acceleration induced noise. As a result, the cutoff frequency of the low-pass filter and hence signal bandwidth can be increased which leads directly to improved spatial resolution of the force measurement along the length of the lumber. This resolution improvement comes without the tradeoff penalties associated with the other means of increasing bandwidth.
The description is directed toward the understanding of a specific application in lumber testing. It is clear that the same solution to noise induced from accelerations is applicable in other force measurement situations where a similar load beam assembly model is valid.